package com.jojo.elementary.day10_tree;

import com.jojo.elementary.entity.TreeNode;

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

/**
 * 145、二叉树的后序遍历
 *
 * 给你一棵二叉树的根节点 root ，返回其节点值的 后序遍历 。
 *
 * 示例 1：
 * 输入：root = [1,null,2,3]
 * 输出：[3,2,1]
 *
 * 示例 2：
 * 输入：root = []
 * 输出：[]
 *
 * 示例 3：
 * 输入：root = [1]
 * 输出：[1]
 *
 * 进阶：递归算法很简单，你可以通过迭代算法完成吗？
 */
public class TreePostErgodic {

    /** myCode 递归 */
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null){
            return list;
        }
        list.addAll(postorderTraversal(root.left));
        list.addAll(postorderTraversal(root.right));
        list.add(root.val);
        return list;
    }

    /** myCode 迭代 */
    public List<Integer> postorderTraversal1(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty()){
            while (root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.right == null || root.right == pre) {
                list.add(root.val);
                pre = root;
                root = null;
            } else {
                stack.push(root);
                root = root.right;
            }
        }
        return list;
    }

    /** pe. Morris遍历 */
    public List<Integer> postorderTraversal2(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }

        TreeNode p1 = root, p2 = null;

        while (p1 != null) {
            p2 = p1.left;
            if (p2 != null) {
                while (p2.right != null && p2.right != p1) {
                    p2 = p2.right;
                }
                if (p2.right == null) {
                    p2.right = p1;
                    p1 = p1.left;
                    continue;
                } else {
                    p2.right = null;
                    addPath(res, p1.left);
                }
            }
            p1 = p1.right;
        }
        addPath(res, root);
        return res;
    }

    public void addPath(List<Integer> res, TreeNode node) {
        int count = 0;
        while (node != null) {
            ++count;
            res.add(node.val);
            node = node.right;
        }
        int left = res.size() - count, right = res.size() - 1;
        while (left < right) {
            int temp = res.get(left);
            res.set(left, res.get(right));
            res.set(right, temp);
            left++;
            right--;
        }
    }
}
